Conventionally, conveyor belt drive systems have been of three types.
A first type works on the basis of direct linkage between an electric motor and an output drive shaft, where there is no fluid coupling therebetween. The torque on the drive shaft is controlled directly by the electric motor and there is no means located along the linkage for further affecting the torque.
The disadvantage of this type of drive system is that it may be necessary to oversize the motor to accommodate the heating up of the motor during acceleration of the belt. An oversized motor adds to the cost.
Also, since most conventional conveyor systems use a plurality of motors spaced apart along the system (known as multimotor drive), a direct linkage or coupling to similarly spaced apart drive shafts means that the respective motor rotor inertias are coupled by the conveyor system itself. This can lead to vibration related problems and difficulty in load sharing in the conveyor system.
A second type of drive system works on the basis of a constant speed electric motor connected through a variable fluid coupling to an output drive shaft. The variable fluid coupling is controllable by means independent of the motor. For instance, the variable fluid coupling may be of the scoop coupling type where the volume of oil within the coupling is varied over time by a scoop means. Variation in the oil volume within the scoop coupling will affect the output torque from the coupling that, in turn, will affect the torque on the output drive shaft. Other examples of independently controllable variable fluid couplings include the scoop trim coupling and the drain and fill coupling. It is a disadvantage of this type of coupling that it is mechanically complex and therefore prone to breakdown.
A third type of drive system includes the Foettinger type fixed-fill fluid coupling. This is the most simple of the three types whereby the coupling is normally driven by a squirrel cage type induction motor at relatively constant speed and produces an output torque which is a function of the coupling design, the type and fixed amount of oil within the coupling and the absolute speeds and relative speeds of the driving and driven parts of the coupling. However, all these variables must be determined prior to operation of the drive system and they remain fixed throughout the operation. It is therefore not possible to alter the characteristic output torque arising from such a coupling during its operation. This is a disadvantage in instances where it is desirable to change output torque during operation of the drive system in response to varying load factors and the like. It is a further disadvantage because the output torque in such drive systems is often less consistent than would be desirable owing to random fluctuations in external variables such as load.
It is an object of the present invention to overcome or substantially ameliorate the disadvantages of the prior art.